Fireclay refractory blocks for use in a tin bath are known in the art. Such blocks have a rectangular cross section, and they include a first surface being adapted for contacting a tin bath, a second surface facing away from the tin bath and four side surfaces. All of these surfaces are grinded to the desired dimensions of the block after producing and firing the block.
A large format fireclay refractory block for use in a tin bath is known from the German Patent No. 42 06 734 C2 and from the corresponding U.S. Pat. No. 5,407,875. The material of the block includes 33 to 43 percent by weight of Al.sub.2 O.sub.3 and 1 to 3 percent by weight of alkali oxide. The material has a resistance to pressure of 35 to 60 N/mm.sup.2, a pressure elastic modulus of 3,000 to 10,000 N/mm.sup.2, an open porosity of 19.7 to 25 percent by volume, a gas permeability of less than 3 nPm and a hydrogen diffusion of less than 1,471.5 Pa (about 147 mm H.sub.2 O). The blocks are placed and mounted in a steal construction being part of a glass production facility. The steal construction is coated by the blocks. Thus, a production tub is formed. The tub is filled with liquid tin. The melted glass is poured out on the surface of the tin bath. The glass expands on the surface of the tin bath, and it is drawn as a thin band having a substantial width over the surface of the tin bath. This is the known way of manufacturing flat glass according to the floating technique. The flat glass includes about 15 percent of Na.sub.2 O. Na.sub.2 O diffuses into the melted metal in the contacting region between the glass and the liquid tin. The tin bath includes sodium and atomic oxygen in a solved manner. The solubility of sodium and of atomic oxygen in tin is a function of the temperature. In the process of producing flat glass, temperatures of about 1,200.degree. C. to 600.degree. C. occur. Due to thermically and mechanically induced floating of the liquid tin in the tin bath, parts of the tin including sodium contact the surface of the blocks facing the tin bath. Thus, an exchange of sodium between the tin and the blocks takes place. The atomic sodium penetrates into the refractory material, and it first reacts with the silica of the glass phase of the fireclay refractory blocks. Thereby, sodium oxide is generated. Due to the reduction of phases including silica, the reduced parts of the blocks show a gray or black color.
The known fireclay refractory blocks for use in the above described glass production facilities may have a length of approximately 1,000 mm, a width of approximately 600 mm and a height of approximately 300 mm. The blocks are made of a material including grains of different sizes of clay and alkali alumo silicate. After firing the block, mainly mullite, a small amount of cristobalite and a glass phase are present. The amount of the glass phase of such a fireclay refractory block or brick is determined by the amount of sodium oxide and potassium oxide. These oxides have a substantial influence on the chemical composition of the glass phase. The ratio of Al.sub.2 O.sub.3 and SiO.sub.2 in the block determines which alkali alumo silicate, either nephelinie or albite, are formed in layers of the block close to the surface of the block in case of an attack by metallic sodium. The thermical expansion factor of nepheline is approximately four times the expansion factor of mullite. Thus, an enlargement and a growing of the layers of the block adjacent to the surface of the block contacting the tin bath occurs. Due to the rectangular shape of the blocks, these layers contact each other. Consequently, tension occurs inside the production tub.
It is necessary not to have great gaps between the grinded side surfaces of the blocks to keep the blocks as close together as possible to prevent the liquid tin from passing through the gaps. Liquid tin passing through the gaps would result in a damage of the steal construction in which the fireclay refractory blocks are arranged. Since a penetration of the liquid tin into the gaps cannot be prevented in all cases, the steal construction is cooled on its outside to freeze tin penetrating through the gaps.
Due to the increase of the volume of the layers in the region of the surface of the block to contact the tin bath, chipping off particles occurs in the region of these layers. These layers are especially located in the edge portions of the surface of the block to contact the tin bath. Since the ceramic material of the fireclay refractory block has a lower relative density than tin, the chipped off material floats upwardly in the direction of the surface of the tin bath. This floating effect of parts of the blocks may cause substantial trouble in the production process of the flat glass.
Another fireclay refractory block is known from the German Patent No. 195 49 057 C1. The surface of the block to contact the tin bath includes a coating including alkali silicate. The coating includes a relative amount of Al.sub.2 O.sub.3 which is decreased compared to the material of the block, and alkali acid and silicid acid being increased with respect to the material of the block. The coating is provided to form a separating layer on the surface of the block, the separating layer forming a glazing and acting as reactant for sodium from the tin bath. With this arrangement, the forming of feldspar or substitutes of feldspar is substantially reduced to prevent chipping off effects in the use of the block in a tin bath. The coating is arranged on the surface of the block contacting the tin bath, this means at least on its upper surface and possibly on portions of the side surfaces of the block. The coating serves to form a thin glazing due to heat. Alkalis are used as flux agents, especially in combination with an increased amount of SiO.sub.2. In the simplest embodiment, the coating is applied by painting and a doctor blade, by spraying or the like. The glazing resulting from the coating preferably has a width of a fraction of a millimeter, but, at the same time, it is to be fixedly anchored in the pores of the surface. The forming of the glazing is possible due to a great amount of alkali acid and silicid acid in the thin layer of the coating. Thus, the glazing itself also has a small width being small enough to prevent chipping off effects and floating effects of pieces of the glazing in the tin bath. The coating and the glazing being formed therefrom act as reactants, and they form a separating layer for atomic sodium delaying reactions, so that the atomic sodium penetrates from the tin bath into the layer of the coating or of the glazing, but not into deeper regions of the material of the block. The effect of the separating layer is also based on the fact that open pores are filled by the coating, and thus, a mechanical separating effect is attained. At the same time, the coating is fixedly anchored in the pores of the surface of the block. Due to a limitation of the coating to a thin layer, layer dimensions are prevented, as they are typical of pieces chipping off and floating in the tin bath in the prior art. Nevertheless, the application of the coating makes an additional step necessary in the production of the block. Thus, the production costs for a block including a coating are increased with respect to a block not including a coating. Additionally, it is difficult to control the thickness of the glazing.